The present invention generally relates to fabrication of semiconductor devices and more particularly to a flow control valve for use in deposition systems used for fabricating a semiconductor device, for controlling supply of gaseous source materials.
In the fabrication of semiconductor devices, various vapor phase deposition processes are employed for depositing a semiconductor layer on a substrate. In such vapor phase deposition processes, a gaseous source material of a desired semiconductor layer is introduced into a reaction chamber, and the deposition of the semiconductor layer is achieved by causing a decomposition of the gaseous source material in the vicinity of the surface of the substrate. As a result of the decomposition, atoms forming the semiconductor layer are released, and the atoms thus released occupy the crystallographic sites of the semiconductor layer that is to be deposited on the substrate. Thereby, the semiconductor layer is grown on the substrate while maintaining an epitaxy with respect to the substrate.
Meanwhile, intensive studies are being made recently on the so-called compound semiconductor devices that employ a compound semiconductor material such as GaAs. These compound semiconductor materials generally have an increased carrier mobility as compared to conventional, single component semiconductor materials such as Si or Ge, and are characterized by the band structure that allows direct transition of carriers. Generally, compound semiconductor materials form a multi-component mixed crystal over a wide compositional range, and techniques are studied to grow semiconductor layers with a composition tailored in correspondence to the specific application of the semiconductor device. For example, formation of a superlattice structure that includes an alternate deposition of GaAs and AlGaAs, is commonly employed in the fabrication of optical semiconductor devices such as laser diodes as well as in the fabrication of quantum semiconductor devices that employs the tunneling effect of carriers. The deposition of such compound semiconductor materials is generally achieved by the metal-organic chemical vapor deposition (MOCVD) process that employs metal-organic source materials.
When forming such a compound semiconductor material, it is necessary to change the composition of the gaseous source material in correspondence to the deposition of each semiconductor layer. Thus, a MOCVD apparatus is required to have a capability of switching the gaseous source material at a high speed and with reliability. Particularly, in the atomic layer epitaxy (ALE) process wherein the semiconductor layer is grown one atomic layer by one atomic layer while changing the composition of the gaseous source material in each atomic layer, it is necessary to switch the composition of the gaseous source material at a very high speed. In the ordinary growth process of compound semiconductor layers, too, incomplete switching the gaseous source material may lead to the formation of undesirable boundary layer between one semiconductor layer and another semiconductor layer. Such a boundary layer has a deviated composition and can cause a detrimental effect on the high speed semiconductor devices such as HEMT that utilizes the property of the heterojunction interface. In view point of increasing the throughput of fabrication, the switching of the gaseous source materials has to be achieved as fast as possible.
FIG.1 shows a typical conventional MOCVD apparatus.
Referring to FIG.1, the MOCVD apparatus includes a reaction chamber 1 in which a semiconductor substrate 1a is disposed for deposition of a desired semiconductor layer such as GaAs or AlGaAs. The reaction chamber 1 is supplied with gaseous source materials via a supply line 1.sub.1, and the gaseous source materials thus supplied release the atoms that form the semiconductor layer, upon decomposition of the gaseous source materials on the surface of the substrate 1a. Upon decomposition, the gaseous source materials are evacuated from the reaction chamber 1 via an evacuation line 1.sub.2 and are released to the environment after processing at a scrubber 3.
In the illustrated example, the MOCVD apparatus includes bubblers 4a-4c for supplying the gaseous source materials, wherein the bubblers 4a-4c hold metal-organic sources such as TMG, TMA, and the like. Each of the bubblers 4a-4c is supplied with a hydrogen carrier gas via a corresponding mass flow controller (MFC), and there occurs a bubbling of the metal-organic sources in the bubblers. As a result of the bubbling, gaseous source materials that may contain Ga or A1 are formed, and the gaseous source materials thus formed are supplied, together with the carrier gas, to the foregoing supply line 1.sub.1 via a gas switching mechanism 2. Similarly, gas cylinders 4d and 4e containing therein a high pressure gas of arsine (AsH.sub.3) or disilane (Si.sub.2 H.sub.6) are connected to the line 1.sub.1 via the gas switching mechanism 2. Thereby, the bubblers 4a-4c and the cylinders 4d and 4e form a gaseous source supplying unit.
It should be noted that the bubblers 4a-4c or the cylinders 4d and 4f supply the gaseous source materials constantly and continuously. Thus, the foregoing gaseous source supplying unit has to be able to switch the flow of the gaseous source materials such that one or more gaseous source materials are supplied directly to the scrubber 3 while circumventing the reaction chamber 1 when the gaseous source materials are not used in the reaction chamber 1 for the deposition of the semiconductor layer. For this purpose, the gas switching mechanism 2 is connected with a purge line 1.sub.3 such that those source materials that are supplied from the gaseous source supplying unit 4a-4e but not used in the reaction chamber 1 are bypassed to the scrubber 1 via the lines 1.sub.3 and 1.sub.2.
The gas switching mechanism 2 includes switching valves 2a-2h, wherein the switching valve 2a has an outlet connected to the foregoing line 1.sub.1 and further to the inlet of the switching valve 2e. Thereby, the switching valve 2a is supplied with a mixture of the gaseous source materials formed at the bubblers 4a and 4b and supplies the mixture selectively either to the line 1.sub.1 or to the inlet of the valve 2e. The valve 2e has an outlet connected to the foregoing purge line 1.sub.3. Similarly, the switching valve 2b is supplied with the gaseous source material formed at the bubbler 4c and supplies the same either to the line 1.sub.1 or to an inlet of the switching valve 2f. Further, the switching valve 2c is supplied with the gaseous source material in the gas cylinder 4d and supplies the same either to the supply line 1.sub.1 or to the inlet of the switching valve 2g, while the switching valve 2d is supplied with the gaseous source material in the gas cylinder 4e and supplies the same either to the supply line 1.sub.1 or to the inlet of the switching valve 2h. It should be noted that each of the valves 2e-2h has the outlet connected to the foregoing purge line 1.sub.3.
In such a construction, the valves 2a-2d form a part of the supply system while the valves 2e-2f form a part of the purge system. Thus, when a valve of the supply system such as the valve 2a is activated, the gaseous source materials formed at the bubblers 4a and 4b are supplied to the reaction chamber 1 via the supply line 1.sub.1. When the valve 2a is deactivated, on the other hand, the gaseous source materials are supplied directly to the scrubber 3 via the purge line 1.sub.3, without passing through the reaction chamber 1.
On the other hand, each of the valves 2e-2h of the purge system is supplied either with the hydrogen carrier gas or with a nitrogen purge gas, and supplies the same to the inlet of the corresponding valve of the supply system. For example, the valve 2e supplies the purge gas to the inlet of the valve 2a, while the valve 2f supplies the purge gas to the inlet of the corresponding valve 2b. Thus, when a valve of the purge system such as the valve 2e is activated, the purge gas flows through the valve 2a and the line 1.sub.1 to the reaction chamber 1 and causes a flushing therein.
In the MOCVD apparatus of the foregoing construction, it is desired to minimize the variation in the flow rate as well as the pressure of the gaseous source materials that flow through the supply line 1.sub.1, even when the state of the valves 2a-2d is switched between the opened state and the closed state. As described previously, such a variation in the supply pressure of the gaseous source material in the reaction chamber 1 may invite a deviation in the composition or other quality of the semiconductor layer that is deposited on the substrate 1a. Thus, the valves 2a-2h forming the gas switching mechanism 2 of FIG. 1 are required to have the capability of controlling the flow rate accurately when switching the gaseous source materials.
Meanwhile, the switching valves 2a-2h of the MOCVD apparatus of FIG.1 are required, in addition to the capability of controlling the flow rate accurately, to have the capability of switching the gaseous source materials at a very high speed. FIG.2 shows an example of such a switching of the gaseous source materials that is employed for conducting an atomic layer epitaxy process. As indicated in FIG.2, the atomic layer epitaxy is achieved by supplying metal-organic source materials such as TMA or TMG like an impulse with an intervening flushing by hydrogen, wherein each pulse duration is preferably set below 1 second.
Conventional MOCVD apparatuses generally use a butterfly valve 5 having a construction of FIG.3(A) for controlling or switching the flow rate of the gaseous source materials. A butterfly valve 5 has a valve box in which a valve chest 5c is provided to extend from an inlet 5a to an outlet 5b. The valve chest 5c is provided with a plate 6 acting as a valve element such that the plate 6 rotates about a rotational axis. Thus, the butterfly valves have a simple construction and can be manufactured with little cost. On the other hand, as shown in FIG.3(B), the butterfly valves have a drawback in that the relationship between the flowrate and the valve position is generally not linear except for very small valve positions in the range of about 20-30%. When the valve position exceeds the foregoing range, the flow control by the butterfly valve is no longer effective. Thus, use of the butterfly valves in the MOCVD apparatus of FIG.1 raises various problems.
FIG.4(A) shows another example of the switching valve used in the MOCVD apparatus, wherein the valve includes a compartment plate 7 disposed to interrupt the flow of the gaseous source material, and a passage 7a of the gaseous source is provided on the compartment plate 7. Further, a flexible diaphragm 8 is provided to close the foregoing passage 7a such that the diaphragm 8 is actuated by an air pressure. Thereby, the switching valve of FIG.4(A) can provide a very high response.
On the other hand, the valve of FIG.4(A) has a drawback in that it cannot control the flow rate as a function of the valve position. Further, as indicated in FIG.4(B), the valve tends to show a delay in operation in that it takes some time from the activation of the valve by the air pressure until the gaseous source material actually starts to flow through the passage 7a. In addition, such an activation of the valve tends to cause an overshoot in the flowrate. Thereby, there is a tendency that an interface layer having a deviated composition is formed at a boundary between one semiconductor layer and another semiconductor layer deposited thereon.